The present application relates to a liquid crystal display panel including a viewing angle control sub-pixel, and particularly to a liquid crystal display panel in which a viewing angle control sub-pixel operates in the FFS mode.
The liquid crystal display panel has features of lighter weight, smaller thickness, and lower power consumption compared with the cathode ray tube (CRT) and therefore is used as a display unit in many electronic apparatuses. The liquid crystal display panel is to display an image by changing the orientation of liquid crystal molecules aligned along a predetermined direction by an electric field to thereby change the amount of light passing through the liquid crystal layer. Among such liquid crystal display panels are a reflective display panel in which ambient light is incident on the liquid crystal layer and is reflected by a reflective plate to pass through the liquid crystal layer again and be output, a transmissive display panel in which incident light from a backlight device passes through the liquid crystal layer, and a semi-transmissive display panel having the characteristics of both the reflective display panel and the transmissive display panel.
As the method for applying the electric field to the liquid crystal layer of the liquid crystal display panel, a method of the vertical electric field system and a method of the transverse electric field system are known. In the liquid crystal display panel of the vertical electric field system, an electric field along substantially the vertical direction is applied to liquid crystal molecules by a pair of electrodes sandwiching the liquid crystal layer. As the liquid crystal display panel of this vertical electric field system, display panels of the twisted nematic (TN) mode, the vertical alignment (VA) mode, the multi-domain vertical alignment (MVA) mode, etc. are known. In the liquid crystal display panel of the transverse electric field system, a pair of electrodes insulated from each other are provided on the internal surface side of one of a pair of substrates sandwiching the liquid crystal layer, and an electric field along substantially the lateral direction is applied to liquid crystal molecules. As the liquid crystal display panel of this transverse electric field system, a display panel of the in-plane switching (IPS) mode, in which the pair of electrodes do not overlap with each other in plan view, and a display panel of the fringe field switching (FFS) mode, in which the pair of electrodes overlap with each other in plan view, are known.
In the IPS-mode liquid crystal display panel, the pair of electrodes as a pixel electrode and a common electrode are formed into a comb-teeth shape so that they may be interdigitated with each other, in such a state as to be electrically insulated from each other, and an electric field along the lateral direction is applied to the liquid crystal between the pixel electrode and the common electrode. This IPS-mode liquid crystal display device has an advantage that the viewing angle is wider than that of the liquid crystal display device of the vertical electric field system.
In the FFS-mode liquid crystal display panel, the pair of electrodes as an upper electrode and a lower electrode are disposed in layers different from each other with the intermediary of an insulating film. Furthermore, slit apertures are provided in the upper electrode and an electric field along substantially the lateral direction passing through the slit aperture is applied to the liquid crystal layer. This FFS-mode liquid crystal display panel is increasingly used in recent years because it has an advantage that a wide viewing angle can be obtained and the image contrast can be improved.
As described above, the liquid crystal display panel of the transverse electric field system has a wide viewing angle. However, when confidential information that is not desired to be seen is displayed, it is preferable to employ a small viewing angle to prevent others from having visual contact with the displayed information. Therefore, as shown in Japanese Patent Laid-open No. Hei 5-108023 (hereinafter, Patent Document 1), there has been known a method of adding a liquid crystal panel for viewing angle control to a liquid crystal panel for displaying and controlling the viewing angle characteristics.
However, this method has a problem that the addition of the panel for viewing angle control greatly increases the thickness of the liquid crystal display panel. As a solution to this problem, as shown in Japanese Patent Laid-open No. 2007-178737 (hereinafter, Patent Document 2) and Japanese Patent Laid-open No. 2009-222747 (hereinafter, Patent Document 3), there has been known a method of adding a viewing angle control sub-pixel in addition to display sub-pixels of red (R), green (G), and blue (B) and controlling the viewing angle characteristics by controlling the voltage applied to the viewing angle control sub-pixel.
With use of FIGS. 7A and 7B, a description will be made below about the configuration of an FFS-mode liquid crystal display panel to which the viewing angle control sub-pixel disclosed in Patent Documents 2 and 3 is added. FIG. 7A is a plan view showing the outline of the array substrate corresponding to one pixel in the FFS-mode liquid crystal display panel to which the viewing angle control sub-pixel of the related arts is added. FIG. 7B is a diagram showing the alignment state of liquid crystal molecules in the viewing angle control sub-pixel.
As shown in FIG. 7A, one pixel 11D of a liquid crystal display panel 10D is composed of a display area 12D and a viewing angle control area 13D disposed adjacent to the display area 12D. The display area 12D is composed of display sub-pixels 16D of three colors of R, G, and B, and the color of the pixel is defined by color mixing of light of these colors. The viewing angle control area 13D includes one viewing angle control sub-pixel 17D. An upper electrode 28 of the liquid crystal display panel 10D operates as the common electrode in this configuration and is formed across all the pixels. A lower electrode 25 operates as the pixel electrode and is formed for each of the display sub-pixels 16D and the viewing angle control sub-pixels 17D.
First slit apertures 29D having a bent shape are formed in the upper electrode 28 in the display area 12D. An alignment film (not shown) is formed on the surface of this upper electrode 28 and the inner surfaces of the slit apertures 29D. For this alignment film, rubbing treatment in the same direction as the extending direction of a signal line 19 (Y-axis direction in FIG. 7A) is performed. The first slit aperture 29D is composed of a first sub-slit aperture 38 inclined to the direction of the rubbing treatment by +6α and a second sub-slit aperture 39 inclined by −6α. In the upper electrode 28 in the viewing angle control area 13D, second slit apertures 30D extended along the direction perpendicular to the direction of the rubbing treatment are formed.
In this liquid crystal display panel 10D, as shown in FIG. 5 in Patent Document 3, liquid crystal molecules in the viewing angle control area 13D are inclined to the surface of the array substrate when a drive voltage is applied between the upper electrode 28 and the lower electrode 25. Therefore, although there is no influence of viewing angle control in the direction of direct viewing of the front face of the liquid crystal display panel 10D, the contrast is lowered due to light leakage and thus viewing the displayed image is difficult in an oblique viewing direction. Consequently, the viewing angle control effect can be exerted.
However, in the liquid crystal display panel 10D of the related arts, the extending direction of the second slit apertures 30D in the viewing angle control area 13D is at a right angle to the direction RD of the rubbing treatment. Thus, as shown in FIG. 7B, when liquid crystal molecules at the center part of the second slit aperture 30D rotate, the direction of the rotation is not settled, which leads to a problem that disclination is generated when viewing angle control is carried out and light leakage occurs also along the direct viewing direction. This light leakage along the direct viewing direction can be suppressed tentatively by blocking light from the center part of the slit by a black matrix. However, in this case, light from the part that effectively has the viewing angle control function originally is also blocked and thus the viewing angle control function is also lowered.